Bacteriophage T7 is a relatively simple virus that has been well characterized physiologically and genetically. Mutations are now available in 25 of the 30 or so T7 genes, including many of the genes involved in replication and maturation of T7 DNA. Mutant strains of E. coli that do not permit normal replication of T7 DNA are also becoming available. Thus, the T7 - E. coli system is an excellent one in which to study the molecular details of DNA replication. Intermediates produced during replication and maturation of T7 DNA will be identified and isolated by electrophoresis through gels of agarose and/or polyacrylamide. Restriction endonucleases that cut T7 DNA at specific sites will be used to determine what portions of the T7 genome are present in such intermediates. Analysis of aberrant forms produced by mutants is expected to provide information about the role of specific T7 and host genes. We are particularly interested in how replication begins during infection by T7 mutants in which the normal origin of replication has been deleted.